1. Field of the Invention
The invention relates to an electronic spark advance control system which times the occurrence of an ignition spark in an internal combustion engine equipped with a precision fuel metering system.
2. Prior Art
Historically the spark used to ignite the air-fuel mixture in the chamber of an internal combustion engine was controlled mechanically. These mechanical devices included an engine driven, multi-lobed cam mounted on a distributor shaft which alternately opened and closed a pair of electrical contacts or points. Opening of the points opened an ignition circuit through an ignition coil which abruptly collapsed its magnetic field and induced a voltage high enough to cause arcing in the spark plugs. Variation in the timing of the spark was also performed mechanically.
Intake manifold vacuum is inversely related to engine load and conventionally sensed by a diaphragm. A change in the manifold vacuum moves the diaphragm and associated linkage to shift the points about the axis of the distributor shaft. In this manner the spark is advanced for a high manifold vacuum level signifying a low load upon the engine.
Mechanical devices have additionally been used to advance the occurrence of the spark in response to changes in engine speed. These devices, functionally similar to a governor, employ weights rotated by the distributor shaft to produce a centrifugal force acting against associated restraining springs. At sufficiently rapid rotation, these weights move the platform supporting the points, causing their shift with respect to the axis of the distributor shaft, thereby advancing the spark in relation to engine speed.
The overall operation of mechanical ignition timing control devices has proved to be different in light of present and proposed federal emission standards. Response delay problems, together with calibration and maintenance difficulties caused by mechanical wear and winding impair performance of mechanical timing control devices increasing emission of pollutants.
Known electronic timing control systems have attempted to reduce inaccuracies occurring during the starting of the engine by providing two independent timing references. One such reference is used to trigger the electronic ignition system and its voltage to time converter when the engine is running. The other independent timing reference is used during starting to directly time the spark without the intervention of a voltage to time conversion. These latter systems require additional mechanical hardware such as a pair of timing wheels or pickups.
In some timing control systems the closing as well as the opening of the points are used to provide two independently adjustable timing points. Such a feature defeats one of the advantages of an electronic timing system; that is, the dwell time of the points becomes an important adjustment requiring a skilled mechanic and additional instrumentation.
Increasingly stringent federal emission and performance standards, together with development of precision fuel metering systems capable of precisely controlling air fuel ratios and cylinder to cylinder distribution of fuel to air mixture, have produced a need for a more reliable and controllable ignition advance system. However, up until the time of the present invention, ignition advance systems of the type described have resulted in higher costs and lower operating reliabilities than are considered to be acceptable.